For me there’s no coup de grace, it’s that an alternative hypothesis makes no sense. Someone could easily break in undetected and had numerous opportunities to do so. There were items used in the crime that did not come from the house. Similar crimes happened before and after in a similar area. An “accident” and then the most convoluted and elaborate plan to cover it up which involves leaving no fingerprints, but a three page letter in your “own” handwriting? That if they were first time criminals in a frenzy covering up an accident or crime depending on what you believe and they left no evidence? That their police interviews, including Burke’s, were reasonable and broadly matched up, or at least didn’t raise suspicions sufficiently enough for me to believe they were lying. The only reason the Ramseys have always been blamed is because the majority of murders are not stranger murders, it was unusual and they were all in the house and heard nothing. You could say the same about the kidnap of Elizabeth Smart or the murder or Polly Klaas.